Allow depot_tools/.git to be a file

This CL relaxes the check for the .git folder on Windows so that it
may be either a file or a folder, as may be the case if depot_tools
is a git submodule or a worktree.

Bug: 1333997
Change-Id: Ib805912903696fc247f85d15c35b6e3093cd57f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3736684
Commit-Queue: Will Cassella <cassew@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>

update_depot_tools.bat

@@ -49,7 +49,7 @@ if errorlevel 1 (
 set errorlevel=
 
 :: Make sure DEPOT_TOOLS_DIR is a git repo
-IF NOT EXIST "%DEPOT_TOOLS_DIR%.git\" (
+IF NOT EXIST "%DEPOT_TOOLS_DIR%.git" (
   echo Error: Your depot_tools directory does not appear to be a git repository, and cannot be updated.
   echo Consider deleting your depot_tools directory and following the instructions at https://www.chromium.org/developers/how-tos/install-depot-tools/ to reinstall it.
   exit /b 1